A simple method has been developed for the assay of enzymes involved in prostaglandin and prostacyclin synthesis. This involves incubation of various fractions from guinea pig lung homogenates with 14C-arachidonic acid and cofactors, GSH and epinephrine. 3H-Prostaglandins (E1 and F2 alpha) were included in the assay mixture to monitor the loss during the assay as well as during the isolation. Prostaglandins extracted with ethyl acetate and purified through a combination of column and thin layer chromatography were quantitated by measuring the radioactivity. PGF 2 alpha and PGE 2 have been characterized by various techniques including reverse isotope dilution as well as radioimmunoassay. Prostacyclin formation was monitored by the formation of 6-keto PGF1 alpha mainly because of the extreme lability of the prostacyclin. A more direct assay of prostacyclin is being developed utilizing the ability of prostacyclin to specifically increase cyclic AMP in platelets.